1. Field of the Invention
The present invention generally relates to multiple access communications control in a Low Earth Orbit (LEO) satellite system composed of satellites that transpond, in real time, transmissions received in the uplink frequency band. These transmissions may or may not be filtered in order to remove unwanted noise before frequency translation and retransmission. It is also assumed that these satellites continually broadcast a beacon on the "outbound" data channel in addition to the "bent pipe" transponder.
2. Background Description
The tracking and location of assets such as railcars, shipping or cargo containers, trucks, truck trailers, automobiles, etc., can be highly advantageous in commerce. Precise tracking of such vehicles and objects can facilitate their being allocated and positioned in an efficient manner, and can provide for immediate, accurate localization of lost, delayed or damaged assets. In one such system, a central facility or station must track multiple assets (e.g., railcars). Each tracked object carries a TAG that contains a transmitter for communicating with a central station.
In another application, TAGs may be attached to widely distributed nodes in a system, such as a power generation system, gas distribution system, and the like. These nodes may produce data or status information that needs to be transmitted to the central station.
The TAGs may be quite remote from the central station that monitors them, and therefore some type of transponder is required to relay the transmissions from the TAGs. The low complexity/modest capacity low earth orbiting satellites, or little LEOs (LLEOs) as they are nomenclated, will play an important role in the transportation and management of many earth bound assets through the TAGs associated with these assets. The key defining measure of the viability of a little LEO/TAG system is the system capacity, as this is the item that can be metered and sold. In turn, one of the key defining parameters of capacity is the communications transport protocol. It is the capacity that determines the upper limit of the revenue value. This protocol must be designed with good and relatively complete knowledge of the user data generation statistics; e.g., how many users are there, how are they geographically distributed, what are their generation rates dependent upon, and so forth. Second, the protocol must be developed knowing the channel. And third, the protocol must be developed to provide adequate error control.
One method of operation is to have the ground based units (i.e., TAGs) transmit when:
i. the satellite is at a maximum elevation with respect to the TAG, and PA1 ii. the satellite is also near a maximum elevation with respect to a ground control station which serves to collect the inbound transmissions from the TAGs and to broadcast the outbound transmissions to the TAGs. PA1 As a satellite comes into common view of a TAG and the central station, the TAG determines: PA1 the TAG monitors the central station outbound link to determine the TAG's eligibility to transmit should it have a message waiting in the transmit queue. This eligibility is based on an allowable window for start of transmission with allowable limits on. PA1 If the TAG has a message to transmit and if it is eligible to transmit during the present satellite pass, the TAG selects a start transmission time at random within the available time window of eligibility. PA1 The central station has a limited number of receiver units which it allocates on a per unit basis to each TAG transmission. If the number of incoming TAG transmissions exceeds this number, then those TAG messages which are not assigned a receiver unit will be lost and further result in a greater multiple access interference (MAI) noise potentially degrading the transmissions processed by the appropriated receiver units. PA1 The central station monitors the TAG transmission rate. If this rate exceeds the capability of the central station to process this rate, the central station reduces the window of eligibility for transmissions and broadcasts this restriction. The central station may control the window of eligibility by allowing TAGs to transmit only if the maximum angle of elevation falls within a tighter set of limits. The central station may also adjust the TAG transmission rate by changing limits of the time window around the time of maximum elevation.
In order for this to work, TAGs must have some means for determining satellite elevation.